The present invention relates to a non-contact measuring method and apparatus for producing a signal representative of a distance between facing surfaces, particularly the air gap between a rotor and a stator of an electric rotating machine.
known in the art is U.S. Pat. No. 4,675,670 (LALONDE et al.), which describes an apparatus and method for measuring an air gap of an electric rotating machine. According to LALONDE et al. a sensor is preferably positioned on the stator of the electric rotating machine. A high frequency signal drives the sensor and a current detector measures a current signal that is inversely proportional to the distance between the rotor and the stator. One drawback of this design is the necessity of providing processing means in order to linearize the current signal. As persons skilled in the art will understand, such a linearization process is undesirable because it diminishes the precision of the measurements. Another drawback of this design is that a transformer must be used in the current detector and it consumes a considerable amount of energy. Yet another drawback of the apparatus is that its size is inadequate for permanently installing it on the rotor. Still another drawback of this apparatus is its inability to easily select the operating frequency. Furthermore, such design does not permit measuring discharges in the stator.
Also known in the art is the HydroScan(trademark) from MCM ENTERPRISE LTD. The HydroScan(trademark) uses a series of sensors which are positioned all around an electric rotating machine for measuring both the distance between rotor and stator and for detecting local partial discharges. The design of the HydroScan(trademark) is undesirable, because it requires expensive and complex equipment which provides readings that must be analyzed by specialists for determining locations of the local partial discharges.
An object of the present invention is to provide a non-contact measuring method and apparatus for producing a signal representative of a distance between facing surfaces of first and second parts, such as the rotor and stator of an electric rotating machine, more precisely, and with a cheaper and less consuming circuit than those of the prior art.
According to the present invention, there is provided a non-contact measuring method for producing a signal representative of a distance between facing surfaces of first and second parts which are closely spaced apart, the second part being grounded, the method comprising the steps of:
(a) positioning a sensor on the surface of the first part, the sensor including first and second parallel, superimposed conductive plates electrically insulated from one another, the plates being parallel to the surface of the first part and electrically insulated therefrom, the second plate being located in between the first plate and the surface of the first part;
(b) feeding the first plate of the sensor with a high frequency signal through a resistance;
(c) feeding an input of a high impedance, low input capacity, unitary gain voltage amplifier with the high frequency signal through the resistance; and
(d) feeding the second plate with an output signal of the amplifier, the output signal of the amplifier being representative of the distance between the two surfaces. Preferably, step (b) is characterized in that the first plate of the sensor is fed through an interior conductor of a coaxial cable; and step (d) is characterized in that the second plate is fed through an external conductor of the coaxial cable.
Preferably, the non-contact measuring method further comprises a step of feeding a demodulator circuit with the output signal of the amplifier, an output signal of the demodulator circuit being a DC voltage signal representative of the distance between the two surfaces.
Preferably, the non-contact measuring method further comprises a step of feeding a synchronous detector with both the high frequency signal and the output signal of the amplifier, an output signal of the synchronous detector being a DC voltage signal representative of the distance between the two surfaces.
Preferably, steps (b) and (c) are characterized in that the resistance is an electrically controlled variable resistance, and the method further comprises the steps of:
feeding an input of a comparator with the output signal of the demodulator circuit;
feeding another input of the comparator with a reference signal;
feeding an up/down input of a counter with an output signal of the comparator;
feeding a clock input of the counter with the high frequency signal; and
feeding a digital control input of the electrically controlled variable resistance with a digital output signal of the counter, the digital output signal being representative of the distance between the two surfaces.
Preferably, steps (b) and (c) are characterized in that the high frequency signal is generated by a frequency controlled generator, and the method further comprises the steps of:
feeding an input of a comparator with the output signal of the demodulator circuit;
feeding another input of the comparator with a reference signal;
feeding an input of a frequency controller with an output signal of the comparator; and
feeding a digital control input of the frequency controlled generator with a digital output signal of the frequency controller, the digital output signal being representative of the distance between the two surfaces.
Preferably, the first and second parts are respectively rotor and stator of an electric rotating machine, the first plate includes a guarded ring, and the method further comprises the steps of:
feeding an input of a high pass filter with the output signal of the amplifier while the rotor is rotating with respect to the stator; and
feeding an input of an amplifier and detector with an output signal of the high pass filter, the amplifier and detector providing a DC voltage signal representative of discharges occurring between internal elements of the stator.
Preferably, the non-contact measuring method further comprises the step of memorizing the DC voltage signal of amplifier and detector as the rotor rotates with respect to the stator to identify amplitudes and locations of the discharges occurring around the stator.
Preferably, the first and second parts are respectively rotor and stator of an electric rotating machine, the first plate includes a guarded ring, and the method further comprises the step of memorizing the output signal of the amplifier while the rotor rotates with respect to the stator to identify amplitudes and locations of air gaps around the stator.
Preferably, the non-contact measuring method is characterized in that:
the high frequency signal used steps (b) and (c) has a frequency between 100 kHz and 500 kHz; and
the resistance used in steps (b) and (c) is substantially 500 kOhm.
According to the present invention, there is also provided a non-contact measuring apparatus for producing a signal representative of a distance between facing surfaces of first and second parts which are closely spaced apart, the second part being grounded, comprising:
a sensor adapted to be mounted on the surface of the first part, the sensor including first and second parallel, superimposed conductive plates electrically insulated from one another, the plates being parallel to the surface of the first part and electrically insulated therefrom, the second plate being located in between the first plate and the surface of the first part;
a high frequency signal generator having an output for generating a high frequency signal;
a resistance connected in series with the output of the high frequency signal generator; and
a high impedance, low input capacity, unitary gain voltage amplifier having an input connected to both the resistance and to the first plate of the sensor, and an output connected to the second plate of the sensor, the output of the amplifier providing an output signal representative of the distance between the two surfaces.